The present invention relates generally to gas turbine engines, and, more specifically, to turbine nozzles therein.
In a gas turbine engine, air is pressurized in a compressor, mixed with fuel in a combustor, and ignited for generating hot combustion gases which flow downstream into a turbine which extracts energy therefrom. The turbine includes a turbine nozzle having a plurality of circumferentially spaced apart nozzle vanes supported by integral outer and inner bands. A high pressure turbine nozzle first receives the hottest combustion gases from the combustor and channels those gases to a turbine rotor having a plurality of circumferentially spaced apart rotor blades extending radially outwardly from a supporting disk.
Overall engine efficiency is directly related to the temperature of the combustion gases which must be limited to protect the various turbine components which are heated by the gases. The high pressure turbine nozzle must withstand the high temperature combustion gases from the combustor for a suitable useful life. This is typically achieved by using superalloy materials which maintain strength at high temperature, and diverting a portion of compressor air for use as a coolant in the turbine nozzle.
Superalloy strength is limited, and diverted compressor air reduces the overall efficiency of the engine. Accordingly, engine efficiency is limited in practice by the availability of suitable superalloys, and the need to divert compressor air for cooling turbine nozzles.
Ceramic materials are being considered for the advancement of turbine nozzles to further increase the temperature capability thereof and reduce the use of diverted cooling air therefor. However, conventional ceramic materials available for this purpose have little ductility and require special mounting configurations for preventing fracture damage thereof limiting their useful life.
Turbine nozzle design is further complicated since the nozzle is an annular assembly of vanes which are subject to three dimensional aerodynamic loading and temperature gradients therethrough. Turbine nozzles expand and contract during operation, with attendant thermally induced stress therefrom.
Monolithic ceramic is readily moldable to form, but is relatively weak at integral junctions thereof. Ceramic Matrix Composite (CMC) introduces ceramic fibers in a ceramic matrix for enhanced mechanical strength. The fibers provide strength in the binding matrix. However, the ceramic fibers have little ductility and therefore have limited ability to bend and match the required transitions in a complex three dimensional component, such as a turbine nozzle.
Accordingly, it is desired to provide an improved turbine nozzle formed of ceramic for withstanding the hostile environment of a gas turbine engine.